1. Field of the Invention
The present invention relates to a flat cathode-ray tube, an electron gun used for the flat cathode-ray tube and a producing method of the gun.
2. Description of the Related Art
Conventionally, in the case of a flat cathode-ray tube, since the depth dimension thereof in a direction for watching a screen panel can be reduced, the flat cathode-ray tubes are preferably used for a portable television set, an in-car television set, a door phone and the like which require thin image receivers for example.
A conventional flat cathode-ray tube is shown in FIG. 1 and FIG. 2.
A flat cathode-ray tube 1 includes a glass tube body 7 comprising a front panel 2, a screen panel 4 formed with a fluorescent surface 3 and a funnel 6 having a neck 5 which are frit-jointed to one another. An electron gun 8 is disposed in the neck 5 of the funnel 6 such that a center axis of the electron gun 8 coincides with a tube axis 11 of the neck 5. A deflection yoke 14 having a horizontal deflection coil 12 and a vertical deflection coil 13 is provided outside from the neck 5 of the glass tube body 7 to the funnel 6. A magnet, a so-called centering magnet 9 for adjusting an electron beam such that the electron beam scans an effective screen, i.e., a fluorescent surface is disposed at a position closer to a front portion of the deflection yoke 14. The centering magnet 9 comprises two ring-like double-pole magnets (permanent magnets) 9a and 9b. 
In the case of the deflection yoke 14, in view of costs and deflection sensibility, a saddle type coil is generally used as the horizontal deflection coil 12 and a toroidal type coil is generally used as the vertical deflection coil 13. An electron beam 15 emitted from an electron gun 13 is deflected in the vertical direction and radiates onto the fluorescent surface 3 of the screen panel 4. The electron beam 15 is deflected symmetrically with respect to the deflection center in the horizontal direction, but is deflected asymmetrically in the vertical direction.
The glass tube body 7 is formed to a flat shape such that the glass tube body 7 becomes longer in the lateral direction in a horizontally defecting direction. The screen panel 4 is disposed in an inclining manner such that the screen panel 4 crosses the tube axis 11 diagonally. An image formed on the screen panel 4 can be seen from the front panel 2. The front panel 2 is transparent and formed in a flat plate-like shape. The flat cathode-ray tube in this case is a reflective type tube. On the contrary, when the image on the screen panel 4 is seen from the side of the screen panel 4, the flat cathode-ray tube is a transparent type tube.
As shown in FIG. 4, the conventional flat cathode-ray tube 1 causes coma aberration which leaves a trail of light behind a luminescent spot on the screen panel 4 like mercury. A beam spot 17 is visually seen with halation, and image quality is degraded.
The present inventors researched a cause of degradation of this beam spot and as a result, they found that a magnetic field due to the centering magnet 9 on the side of the neck influences the beam spot. That is, by the effect of the magnetic field from the centering magnet 9, as shown in FIG. 3, the electron beam 15 is deflected before the beam 15 enters a main lens 16M, and the electron beam 15 is separated from the tube axis 11, i.e., a so-called xe2x80x9caxis-separationxe2x80x9d is generated. Since the axis-separation is generated on the side of a cathode K of the main lens 16M, the electron beam 15 radiates onto a position deviated from a center O of the main lens 16M. Therefore, the coma aberration is generated, the beam spot 17 attended with halation is generated, which degrades the image quality.
In view of the above circumstances, the present invention provides a flat cathode-ray tube, an electron gun used for the flat cathode-ray tube and a producing method of the gun capable of reducing the degradation of a beam spot caused by effect of a magnet.
A flat cathode-ray tube according to the present invention includes a magnet outside of a neck, and a prefocus lens of an electron gun is separated from the tube axis.
According to the flat cathode-ray tube, since an axis of the prefocus lens is separated in an opposite direction in correspondence with an axis-separating amount of the electron beam whose axis is separated by effect of the magnet outside the neck, the electron beam passing through the focus lens is moved in a direction opposite to the axis-separating direction caused by the magnet, the axis-separation and the axis-separating amount are offset by each other, and the electron beam passes through a center of the main lens.
An electron gun for a flat cathode-ray tube of the present invention comprises a cathode and a plurality of grids, characterized in that a prefocus lens is separated from a center axis of an electron gun in a direction in which an axis-separating amount of an electron beam caused by a magnetic field of a magnet which is disposed outside of a neck becomes smaller.
According to the electron gun for a flat cathode-ray tube of the invention, the prefocus lens is separated from the center axis of an electron gun in a direction in which the axis-separating amount of the electron beam caused by the magnetic field of the magnet which is disposed outside of the neck becomes smaller. Therefore, when the gun is used for the flat cathode-ray tube, the electron beam passing through the focus lens is moved in a direction opposite to the axis-separating direction caused by the magnetic field of the magnet, the axis-separation and the axis-separating amount are offset by each other, and the electron beam passes through a center of the main lens.
A producing method of an electron gun for a flat cathode-ray tube according to the present invention comprises the steps of: preparing a first grid having an electron beam through hole formed at a reference position and having a positioning hole formed at another reference position, and preparing a second grip having an electron beam through hole separated from a reference position by a predetermined distance and having a positioning hole formed at another reference position, and inserting positioning means in the positioning holes of the first and second grids for positioning the first and second grids in a state that a spacer is interposed between the first and second grids.
According to the producing method of the electron gun for the flat cathode-ray tube of the invention, the electron beam through hole of the second grid is previously separated from the reference position by a predetermined distance, and the first and second grids are positioned by the positioning means through the spacer therebetween. Therefore, it is possible to easily and precisely produce an electron gun which is formed such that the prefocus lens can correct the axis-separation of the electron gun.
Another producing method of an electron gun for a flat cathode-ray tube of the invention comprises the steps of: preparing a first grid having an electron beam through hole formed at a reference position and having a positioning hole formed at another reference position, and preparing a second grip having an electron beam through hole formed at a reference position and having a positioning hole formed at another reference position, and inserting positioning means in the positioning holes of the first and second grids for positioning the first and second grids such that an end surface having an electron beam through hole of the second grid is inclined with respect to the first grid in a state that a tapered spacer is interposed between the first and second grids.
According to the producing method of the electron gun for the flat cathode-ray tube of the invention, the first and second grids are positioned by the positioning means through the tapered spacer therebetween. Therefore, it is possible to easily and precisely produce an electron gun which is formed such that the prefocus lens can correct the axis-separation of the electron gun.